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Configuring Basic MX-series Layer 2 Features
You configure MX-series routers exactly as you would any other router
running the JUNOS software. That is, all the familiar Layer 3 (L3)
features and protocols are available on the MX-series routers. However,
you can configure Layer 2 (L2) features that are unique to the MX-series
routers. This chapter addresses L2 configuration for the MX-series
routers. For information about configuring L3 features and protocols,
as well as comprehensive information about interfaces and system basics,
please see the other JUNOS configuration guides.
Configuring L2 features on an MX-series router can vary from
the very simple (aggregated Ethernet trunk interfaces, spanning trees),
to the more complex (inner and outer VLAN tags, broadcast domains),
to the very complicated (integrated bridging and routing, L2 filtering).
This chapter offers a fairly complex configuration for L2 processing
in a bridged environment.
Generally, there are four things that you must
configure in an L2 environment:
- Interfaces and virtual LAN (VLAN) tags—L2 interfaces
are usually various type of Ethernet links with VLAN tags used to
connect to customer devices or other bridges or routers.
- Bridge domains—Bridge domains limit the scope of
media access control (MAC) learning (and thereby the size of the MAC
table) and also determine where the device should propagate frames
sent to broadcast, unknown unicast, and multicast (BUM) MAC addresses.
- Spanning Tree Protocols (xSTP, where the “x”
represents the STP type)—Bridges function by associating a MAC
address with an interface, similar to the way a router associates
an IP network address with a next-hop interface. Just as routing protocols
use packets to detect and prevent routing loops, bridges use xSTP
frames to detect and prevent bridging loops. (L2 loops are more devastating
to a network because of the broadcast nature of Ethernet LANs.)
- Integrated bridging and routing (IRB)—Support for
both Layer 2 bridging and Layer 3 routing on the same interface. Frames
are bridged if they are not sent to the router's MAC address. Frames
sent to the router's MAC address are routed to other interfaces configured
for Layer 3 routing.
Consider the network in Figure 5. The figure shows three MX-series routers acting
as L2 devices.
Figure 5: Bridging Network with
MX-series Routers
The network
administrator wants to configure these links and devices so that:
- The six Gigabit Ethernet links between Router 1 and the
other routers (ge-2/1/0 through ge-2/1/5) are gathered
into two aggregated Ethernet (AE) links mixing bridged traffic from
the VLANs. AE1 will consist of the first three links and AE2 will use the last three links. The same approach is taken
for the links on Router 2 and Router 3.
- The Gigabit Ethernet links from Router 1 to the customer
devices (ge-2/2/1 and ge-2/2/6 ) will be bridged
and include VLAN tag 100 on ge-2/2/1 and VLAN tag 200 on ge-2/2/6. The other two routers, Router 2 and Router 3, also
have two ports configured to handle VLAN 100 on one port (ge-2/2/2) and VLAN 200 on the other (ge-3/3/3).
- The routers have bridge domains reflecting these VLAN
configurations.
- Because the VLANs appear on each MX-series router, the
routers run Multiple STP (MSTP) on the links connecting them to prevent
bridging loops (Rapid STP, or RSTP, does not recognize VLAN tags and
blocks ports without regard for VLAN tagging).
- Router 2 and Router 3 have IRB configured so that they
can pass traffic to other routers in the rest of the network. These
interfaces are configured in Configuring Integrated Bridging and Routing.
- Router 1 has an access interface which provides bridging
on VLAN 205 and is connected to a customer device configured on ge-2/2/2 . Router 3 has an access interface which provides bridging
on VLAN 200 and is connected to a customer device configured on ge-2/2/6.
- Router 1 and Router 3 are configured with a trunk interface
to a switch for VLANs 200–205. On both routers, this interface
is ge-2/2/4.
This chapter provides the following information
about this MX-series L2 configuration of the three routers:
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